Sex may influence the mediating effect of honesty in the relationship between serotonin and reaction to unfairness.

In a recent article, Takahashi et al. (1) described interesting work regarding the mediating effect of honesty in the relationship between serotonin and reaction to unfairness. After reading the article with interest, we have some concerns about the study. First, only male subjects were investigated in their study (1). Sex differences in the serotonin system and personalities would potentially confound the results. Although the authors noted that sex could influence the reaction to unfairness, they ignored the sex differences in the serotonin system. Several studies have demonstrated sex differences in the serotonin system (2). Besides, sex differences in the rate of serotonin synthesis in the healthy human brain have been reported by Nishizawa et al. (3). Second, a number of studies have documented sex differences in personalities. In addition, sex and age differences in trust were reported. Previous studies showed aggressive personalities (i.e., impulsivity and anger–hostility) were related to a high rejection rate of unfair offers in an ultimatum game, which Takahashi et al. (1) mentioned in their study. Additionally, Parsey et al. (4) demonstrated that serotonin and sex have been implicated in aggression. The complicated relationship among sex, serotonin, and aggression should be addressed, which may potentially confound the results. Takahashi et al. (1) ignored the sex differences in trust and aggression. Finally, the Sobel test used in the study (1) may be not appropriate because the sample size (N = 20) was small. The Sobel test has been shown to have low power in small sample sizes: according to MacKinnon et al. (5), sample sizes of at least 50 in single-mediator models will show low bias. Bootstrapping has been shown to work better, and we wonder if the authors could test the mediating effect by using this method. An alternative method may be to enlarge the sample size and use the structural equation model to test for mediation. Zhibing Yang, Danmin Miao, Xufeng Liu, and Xia Zhu Department of Psychology, School of Aerospace Medicine, Fourth Military Medical University, Xi’an 710032, People’s Republic of China